Much of the effort to develop a high temperature superconducting (HTS) wire or tape has focused on coated conductors based on the epitaxial growth of high temperature superconducting (HTS) films on tapes that possess a biaxially-textured surface. Superconducting films with critical current densities in excess of 1 MA/cm2 at 77 K and self-field have been achieved for epitaxial YBa2Cu3O7 films on biaxially-textured tapes produced either by ion-beam assisted deposition (IBAD) or thermomechanically-textured metals.
In previous work involving IBAD, the synthesis of the biaxially-textured buffer layer suitable for HTS films capable of carrying high critical current densities has employed the ion-assist process to produce both the in-plane and out-of-plane texture. In order to realize an HTS film possessing a high critical current on a biaxially textured substrate, the buffer layer architecture must satisfy rigorous requirements. The grains within the topmost buffer layer construct must generally provide a common in-plane and out-of-plane crystallographic texture with a mosaic spread of generally less than 20°, with lower mosaic spreads such as less than 10° providing better superconducting articles.
The top layer must also be chemically compatible with the superconductor so as to not react during superconductor deposition and be mechanically robust to prevent microcrack formation at the HTS/buffer layer interface. To date, the only IBAD buffer layers that have met these objectives have required the use of the ion-assist process in determining the in-plane and out-of-plane texture. For example, biaxially textured yttria-stabilized zirconia (YSZ) buffer layer can be formed with the (100) in-plane and (001) out-of-plane texture by directing an Ar+ beam flux oriented 55° from the surface normal, which corresponds to the [111] direction for a (001)-oriented cubic material.
A significant limitation for the above-described IBAD process is that the optimal biaxial texture requires a relatively thick (>1 μm) YSZ film deposited in the presence of the Ar+ beam. This makes the process relatively slow and as a result expensive, which is a significant issue in the large-scale production of superconducting tapes. A second approach involves the IBAD deposition of MgO requires a sub-10 nm control of the nucleation process, typically employing an in-situ monitoring technique, such as reflection high energy electron diffraction, for controlling the crystallographic texture. This approach is difficult to employ for large-scale production. Also, the quality of MgO films deposited by IBAD has been found to be extremely sensitive to minor variations in the processes and structures used for this material.